In recent years, along with widespread use of portable personal computers and portable phones, it has been highly demanded to develop electricity storage devices, such as a lithium ion secondary battery, having higher capacity and a reduced size. However, existing electricity storage devices, such as a lithium ion secondary battery, mainly use an organic electrolytic solution as an electrolyte. The organic electrolytic solution exhibits high ion conductivity, but has possibilities of leakage, ignition, and the like when used for the electricity storage devices because the organic electrolytic solution is a liquid and flammable.
In order to solve the above-mentioned problem and secure intrinsic safety, an all-solid-state battery which uses a solid electrolyte instead of the organic electrolytic solution and whose positive electrode and negative electrode are each formed of solids has been developed. Such all-solid-state battery uses the solid electrolyte, and hence has no concerns of ignition and liquid leakage, and is less liable to cause problems of deterioration in battery performance owing to corrosion, and the like. Of the all-solid-state batteries, an all-solid-state lithium ion battery has been actively developed in various fields (for example, see Patent Literature 1).
Meanwhile, with regard to lithium to be used for the all-solid-state lithium ion battery, there are concerns globally about price increase and depletion of its raw material. Attention has been focused on a sodium ion as an alternative to a lithium ion, and also an all-solid-state sodium ion battery has been developed (for example, see Patent Literature 2).